1.0 Field Of The Invention
This invention relates generally to dishwashing machines, and more particularly to methods and apparatus for supplying the washing tank of a dishwashing machine with chemical substances.
2.0 Discussion Of Related Art
In institutional dishwashing machines, with which the present invention is particularly concerned, fresh water is supplied to washing tank--also known as the dosing tank--in the clear-rinse phase. The corresponding quantity of detergent--controlled through conductivity--is subsequently added as required. Alkaline detergents of the described type used in practice contain active chlorine as their bleach component, mostly in the form of liquid chlorine bleach in the case of liquid detergents or in the form of di- and trichloroisocyanurates in the case of powder-form detergents.
The use of active chlorine as bleach in dishwashing machines is ecologically objectionable. As a result, efforts are being made to use environment-friendly bleaches. Considerable difficulties have been encountered in this regard. At the present time, only active oxygen is available as an economically reasonable alternative to active chlorine. However, this bleaching agent cannot be formulated with the generally highly alkaline liquid detergents.
Although solid active oxygen carriers can be incorporated in powder-form alkaline detergents, corresponding detergent formulations quickly lose their bleaching effect after flushing into the washing tank of the dishwashing machine because the active oxygen content of the rinse liquor quickly diminishes--often at around 40 to 60% per hour--during machine stoppage times. When a new rinse cycle begins, insufficient active oxygen is available in the washing tank so that cleaning performance can be unsatisfactory at the beginning of the rinse cycle.
EP-A-0 195 619 describes a method in which rinse cycles are carried out with a substance containing active oxygen after one or more wash cycles in a dishwashing machine comprising a spray unit.
In addition, U.S. Pat. No. 4,373,863 describes an apparatus for supplying the washing tank of a dishwashing machine with chemical substances which consist of a detergent dosing system for supplying detergent and a separate dosing system comprising a pump and a storage container for supplying another substance.
3.0 Summary Of The Invention
The problem addressed by the present invention was to enable active oxygen to be used as bleach in conjunction with an alkaline liquid detergent and to eliminate any significant reduction in the active oxygen content in the washing tank during pauses in dosing, even where a bleach-containing powder-form detergent is used. For the method mentioned at the beginning, the solution provided by the invention is characterized in that, where active oxygen is used as the bleach component, the active oxygen content in the washing tank is maintained during pauses in the dosing of detergent by maintenance dosing of the bleach. The maintenance dosing preferably takes place in individual dosing strokes.
Accordingly, one embodiment of the invention relates to maintenance dosing of the bleach during the stoppage times of the dishwashing machine between two successive rinse cycles. The effect of this maintenance dosing is that fresh bleach containing active oxygen enters the washing tank as the active oxygen content diminishes. Accordingly, the washing tank of the dishwashing machine is always kept in readiness for a new rinse phase and is therefore able immediately to supply dishwashing detergent containing sufficient bleach at the beginning of the rinse phase.
Another embodiment of the invention is characterized in that, where a detergent, particularly a powder-form detergent, basically containing sufficient bleach for immediate rinsing is used, bleach continues to be introduced into the washing tank as the active oxygen content decreases during the machine stoppage times or pauses in the dosing of detergent.
In addition to this maintenance dosing regime, a parallel bleach dosing regime is preferably provided in cases where a bleach-free detergent, particularly a liquid detergent, is used. According to another aspect of the invention, this means that, where a bleach-free detergent is used, bleach is introduced into the washing tank commensurately with detergent consumption during the rinse phases and commensurately with the reduction in the active oxygen content during the stoppage phases or the pauses in the dosing of detergent. Accordingly, the parallel dosing regime mentioned provides for the use of active oxygen carriers, such as for example hydrogen superoxide (H.sub.2 O.sub.2) and/or peracetic acid solution, which cannot be formulated with the usual bleach-free alkaline detergents.
According to the foregoing, liquid or powder-form detergent is only introduced or flushed in during certain dosing times in dependence upon the conductivity control or the like, even during the rinse phases. If the corresponding intervals or the pauses in the dosing of detergent become so long that the active oxygen content decreases to the extent where the cleaning performance of the next rinse phase is impaired, bleach continues to be introduced, again by maintenance dosing, commensurately with the reduction in the active oxygen content, even in intermediate or standard pauses in the parallel dosing occurring during the rinse phases.
In another embodiment of the invention, the bleach input capacity of the maintenance dosing is optimized by determination of the active oxygen content. Once the rate of decomposition of the active oxygen is known, it can be sufficient if the bleach continues to be introduced at certain time intervals. In this connection, it is preferred to commence introduction of the bleach by maintenance dosing after the decomposition of about 20% of the active oxygen present.
In an apparatus for supplying the washing tank of a dishwashing machine with chemical substances, consisting of a detergent dosing system for supplying detergent and a separate dosing system comprising a pump and a storage container for supplying another substance, the solution provided by the invention is characterized in that the other dosing system is a bleach dosing system which has an operational mode for maintenance dosing which is activated during the pauses in the dosing of detergent.
The detergent dosing system may be designed in the usual way, i.e. as a dosing pump in the case of a liquid detergent or as a fresh water or liquor flush-in system in the case of a powder-form detergent. According to the invention, therefore, the only addition is the bleach dosing system separate from the detergent dosing system. This provides for parallel dosing at the same time as dosing of the bleach-free detergent and provides for maintenance dosing during the stoppage times of the dishwashing machine or the pauses in the dosing of detergent so that the consumption of active oxygen during such periods is compensated.
This auxiliary or maintenance dosing is necessary if, as is usually the case, the dishwashing machine is supplied with detergent via a control system which does not take the bleach content into account. This is because, in contrast to active oxygen, the concentration or quantity of detergent remains constant during the stoppage phases. Without the maintenance dosing of bleach, only detergent would be introduced into the washing tank at the beginning of the rinse phase commensurately with the inflow of fresh water. Accordingly, there would be a deficit of active oxygen and the outcome of the dishwashing process would be unsatisfactory, at least in regard to bleachable soil. According to the invention, these problems are overcome by the addition of a bleach dosing system.
According to the foregoing, there are in particular two possible embodiments of the invention, namely: (1) either the bleach dosing system has only one operating mode for maintenance dosing, i.e. when bleach-containing detergent is present, or (2) the bleach dosing system has an additional operating mode--functioning in parallel with the detergent dosing system--for parallel dosing when the detergent dosing system is switched on in cases where a bleach-free detergent is used. Accordingly, both bleach-containing and bleach-free detergent can be used, bleach only having to be introduced once during the entire period of operation of the dishwashing machine and, in the other case, only during the stoppage times between two rinse phases or only during dosing of the detergent. Essentially, parallel and maintenance dosing only differ in the bleach dose, i.e. in the quantity of active oxygen introduced into the washing tank per unit of time.
In another embodiment of the invention, a frequency-controlled peristaltic pump may be used for the parallel or maintenance dosing regime of the bleach dosing system. A pump of this type can be operated with a number of pump strokes per unit of time corresponding to the decomposition of active oxygen. For maintenance dosing, relatively few pump strokes are sufficient whereas a much larger number of pump strokes is necessary for parallel dosing where a bleach-free detergent is used. Accordingly, the associated frequency control of the pump preferably comprises a first control range for maintenance dosing and, optionally, a second control range for the rinse phase of the dishwashing machine--when the detergent dosing system is switched on--with a considerably increased input capacity in relation to the dose of the first control range.